Abstract: Vanadium redox flow battery (VRFB) stands as a highly encouraging technology for storing renewable energy, but its performance is limited by the poor hydrophilicity and insufficient active sites of graphite felt (GF) electrodes. This study synthesized metal-nitrogen-carbon materials M-N-C (M = Fe, Co, Ni, and CoFe) via an in-situ growth method to modify GF electrodes. The resulting electrodes exhibit a large specific surface area, stable catalyst loading, excellent hydrophilicity, and abundant active sites, facilitating vanadium ion adsorption and enhancing redox reaction kinetics. Transition metals coordinate with nitrogen to form M-N_x active sites, where modulation of the d-electron orbitals optimizes the adsorption energy of reaction intermediates, consequently reducing the reaction energy barrier. M-N-C@GF (M = Fe, Co, Ni, and CoFe) greatly promote the redox reaction kinetics for VO²⁺/VO₂⁺ and V³⁺/V²⁺, with the order being CoFe-N-C@GF > Co-N-C@GF > Fe-N-C@GF > Ni-N-C@GF. Theoretical calculations prove that these materials have low adsorption energy for vanadium ions and good electron transfer characteristics. The initial energy efficiency (EE) of the CoFe-N-C@GF cell reaches 85.60 %, representing a 5.78 % increase over GF cell at 50 mA cm^-2. Importantly, it can operate for 400 cycles at 150 mA cm^-2. Introducing M-N-C materials into VRFB provides new ideas for the modification of future GF.

Key words: Vanadium redox flow battery, Metal-nitrogen-carbon, Electronic structure regulation, Composite electrode, Redox reactions kinetics